The present invention relates to a hydraulic elevator in which a car is controlled by a control of a flow rate of fluid flowing to or from a hydraulic jack for driving the car through a control of rotational speed of a hydraulic pump, and to a control method thereof.
In this kind of hydraulic elevator, there is a known method of controlling a flow rate of pressurized fluid through a control of rotational speed of a hydraulic pump according to a speed instruction by an electric motor. In particular, as electric control apparatus and control technique progress, a control of rotational speed through control of a motor by an invertor has been easy and reliability of such a hydraulic elevator that a rotational speed thereof is controlled by an inverter-driven motor has been raised. In this kind of conventional hydraulic elevator, its main point is a speed control so as to carry out smooth running of a car. In the elevator, a fluid control valve is required to provide excellent comfort for a of person in the car, to reduce pressure loss in usual running and to provide all safety functions, such as keeping the car at the stop position by rapid closing of the fluid control valve immediately after the car stops.
Thus, the fluid control valve has been required of various functions. An example of such hydraulic elevator is disclosed in JP A 57-81073(1982).
In a hydraulic elevator in which the present invention is applicable, safety security of the car depends on a fluid control valve. In normal running, the fluid control valve is closed rapidly to prevent the car from sinking due to fluid leakage at the hydraulic pump after the car stops. On the other hand, the car may run at a higher speed than an allowable limit when the hydraulic pump is driven at a higher speed than a rated speed due to a control apparatus error or when the pump has no power supplied thereto due to a power failure. In such a case, the car should stop safely at a shortest distance from a position that the car is braked, with a small braking shock.
Even during emergency, different functions are required for the control valve to control the speed of the car during the emergency. When the car is running at a high speed, rapid closing of the control valve increases braking shock. Therefore, the control valve is required to close at a suitable speed. Since breaking occurs abruptly, persons in the car cannot brace themselves for protection. Accordingly, the braking shock should be reduced to prevent the risk of injury.
When the car runs at a relatively low speed, rapid closing of the control valve is required to prevent acceleration of the car due to release of the braking force or to shorten the braking distance. In particular, since a door of the car is opened during leveling of the car to the floor, it is necessary to drastically shorten the braking distance.
As mentioned above, the control valve has specific characteristics for normal operation and emergencies, with the characteristics during an emergency depending on the car operating sped at the time of the emergency.